1. Field of the Invention
This invention relates to the production of hot melt coating compositions which compositions contain microcapsules and inorganic pigment particles dispersed in a hydrophobic hot melt suspending medium. In a preferred embodiment, these hot melt coating compositions may be applied to a substrate such as paper to produce pressure-sensitive carbonless transfer sheets.
2. Prior Art
It is known from the prior art to use hot melt coatings containing microcapsules for coating substrates such as paper. See, for example, U.S. Pat. No. 3,016,308 (1962) to Macaulay and U.S. Pat. No. 3,079,351 (1963) to Staneslow et al. These patents disclose the use of such coatings in the preparation of pressure-sensitive copying papers.
Pressure-sensitive carbonless copy paper, briefly stated, is a standard type of paper wherein during manufacture the backside of the paper substrate is coated with what is referred to as a CB or transfer coating, the CB coating containing one or more chromogenic materials generally in capsular form. At the same time the front side of the paper substrate is coated during manufacture with what is referred to as a CF coating, which contains one or more chromogenic materials capable of producing a color with the encapsulated CB chromogenic material. Both of the chromogenic materials remain in the coatings on the respective back and front surfaces of the paper in substantially colorless form. This is true until the CB and CF coatings are brought into overlying relationship and sufficient pressure, as by a typewriter, is applied to rupture the CB coating to release the encapsulated chromogenic material. At this time the chromogenic material contacts the CF coating and reacts with the chromogenic material therein to form a colored image. Carbonless copy paper has proved to be an exceptionally valuable image medium for a variety of reasons, only one of which is the fact that until a CB coating is placed next to a CF coating both the CB and CF coatings are in an inactive state as the coreactive elements are not in contact with one another until pressure is applied. Patents relating to carbonless copy paper products are:
U.s. pat. No. 2,712,507 (1955) to Green PA0 U.s. pat. No. 2,730,456 (1956) to Green et al PA0 U.s. pat. No. 3,466,184 (1969) to Bowler et al PA0 U.s. pat. No. 3,672,935 (1972) to Miller et al
U.S. Pat. No. 3,455,721 (1969) to Phillips et al
More recently, improved hot melt systems have been developed which can be used to prepare, for example, pressure-sensitive transfer sheets. Such improved systems are described in commonly-assigned, co-pending U.S. application Ser. No. 747,682, filed Dec. 6, 1976. The improvements described therein involve the use of specified materials, e.g. use of polar waxes and/or hot melt suspending media containing polar groups, in order to improve the dispersion of the microcapsules in the hot melt suspending media. The optional use of opacifying agents, such as titanium dioxide or clay, as an additive to the hot melt is mentioned. It should be noted that in some instances the coating compositions were an off white or light tan colored. This is objectionable particularly where the coating compositions are to be applied to white papers. Efforts to overcome this disadvantage by adding pigment particles to the hot melt coating composition were unsatisfactory due to the formation of lumps of pigment particles which could not be easily dispersed.
The treatment of pigments with various organic compounds to improve their dispersability in coating compositions, both hydrophobic and hydrophilic, is well known in the prior art. A review of some of the patents concerned with this solution to the problem of dispersing pigments is disclosed in U.S. Pat. No. 3,640,792 (1972) to Dietz. Dietz also discloses that a slurry of titanium dioxide can be spray dried in the presence of proton-donating organic compounds, e.g., organic amines and amides. The titanium dioxide so treated can be dispersed in a hydrophobic liquid such as a linseed oil vehicle.
The aforementioned objections and disadvantages can be easily overcome using the process of the present invention in which pigment particles are dispersed in an aqueous dispersion of microcapsules and the dispersion of microcapsules and pigment particles are spray dried to form a fine powder. The powder can then be easily dispersed in the heated hot melt coating composition using low shear mixing to give a smooth coatable composition. Sufficient pigment particles can be introduced in this manner to the hot melt coating composition to mask color of the hot melt suspending media. Furthermore, separate treatment of the pigment particles to improve their dispersability in a hot melt suspending medium is not required.